I had this some years ago and it was due to worn bores/piston rings. When
the engine is 'cold' say on a short run the viscosity of the oil in the sump
was high enough to resist the pressure leaking past the pistons and was not
blown out of the crankcase breather. If used on a long run then the oil
would get blown out and this would happen over a small rise in oil
temperature. The apparent performance of the engine did not seem to be
affected. It worth noting that small rises in temp (20C)can reduce the
viscosity of oil by half
The cure was rebore and new pistons.
It would be worth checking that if you have bought this engine second hand,
that all the pistons have all their rings, as I have seen engines with the
rings missing altogether.

Hope this helps.....ttfn......Alistair